CodePen is a social development environment for front-end designers and developers.
Stop wasting time setting up a development environment. Repl.it gives you an instant IDE to learn, build, collaborate, and host all in one place.
The MDN Web Docs site provides information about Open Web technologies including HTML, CSS, and APIs for both Web sites and progressive web apps. It also has some developer-oriented documentation for Mozilla products, such as Firefox Developer Tools.
A handy HTML reference guide.
A handy CSS reference guide.
Netlify is everything you need to build fast, modern websites: continuous deployment, serverless functions, and so much more.